You are a marine for the government sent into a portal to stop an enemy known as "Quake". This enemy has been sending terrible demons and death squads through the government's new slipgate technology. This slipgate technology allows instant transportation of cargo or any other material. Once sent through the portal you must fight through hundreds of demons to stop the enemy. The other realm is inspired by several influences, notably that of H. P. Lovecraft (the end game boss being Shub-Niggurath herself).